The United States has expressed concern to Moscow about what it called reports of an imminent and enhanced Russian military build-up in war-torn Syria.

Washington made its views known in a telephone call from secretary of state John Kerry to his Russian counterpart Sergei Lavrov, the US State Department said.

"The secretary made clear that if such reports were accurate, these actions could further escalate the conflict, lead to greater loss of innocent life, increase refugee flows and risk confrontation with the anti-IS coalition operating in Syria," the department said.

"The two agreed that discussions on the Syrian conflict would continue in New York later this month," it said.

The New York Times reported that Russia has sent a military advance team to ally Syria and was taking other steps that Washington fears may signal plans to vastly expand its military support for president Bashar al-Assad.

The Times said the moves included the recent transport of prefabricated housing units for hundreds of people to a Syrian airfield and the delivery of a portable air traffic control station there.

Russian president Vladimir Putin was asked on Friday whether Russia was taking part in military operations against the Islamic State group in Syria.

"We are looking at various options but so far what you are talking about is not on the agenda," he said.

"To say we're ready to do this today -- so far it's premature to talk about this. But we are already giving Syria quite serious help with equipment and training soldiers, with our weapons," RIA Novosti state news agency quoted Mr Putin as saying.

Russia's foreign ministry said Saturday's telephone call was made at Mr Kerry's initiative.

It made no mention of US concerns about a possible Russian military build-up, but said the two men discussed "different aspects of the situation in Syria and its environs, as well as the objectives of the fight against IS and other terrorist groups".

The foreign ministry said they spoke of "cooperation" between Moscow and Washington to "support UN efforts aimed at launching a political process in Syria".

Mr Lavrov and Mr Kerry agreed to remain in close contact in pursuing a settlement of the Syrian conflict, which has claimed more than 240,000 lives since March 2011 and driven millions more from their homes.

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